The Impact of Exceptional Events 

‘Unusual Winds’ on PM10 

Concentrations in Arizona

Air Quality Division 

October 15, 2009
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TABLE OF CONTENTS 

Page

1.0 INTRODUCTION....................................... ....................................................................... 1 

2.0 SPATIAL AND TEMPORAL VARIABILITY OF WINDBLOWN DUST EVENTS...............1 

3.0 THRESHOLD FRICTION VELOCITIES AND THRESHOLD WIND SPEEDS...................3 

 

4.0 BEST AVAILABLE CONTROL MEASURES (BACM) EFFECTIVNESS.........................9 

 

5.0 OTHER INFLUENTIAL FACTORS IN THE FORMATION AND IMPACT OF..................9 

 WINDBLOWN DUST 

6.0 THE EFFECT OF PARTICLE SIZE AND TURBULENCE ON DUST SETTLING..............10 

 RATES 

7.0 LITERATURE CONCLUSION............................................................................11 

8.0 EXAMINATION OF HISTORICAL WIND & PARTICULATE MATTER........................11 

 DATA IN ARIZONA – WIND THRESHOLD EFFECT 

9.0 UNUSUAL WINDS..........................................................................................15 

LIST OF TABLES Page

Table 1 Frequency of Dust Episodes from Various Studies for Yuma, AZ.............................2 

Table 2 Threshold Friction Velocities for Typical Desert Surface Types................................4 

Table 3 Aerosol Emission Test Results............. .......................................................................... 5 

Table 4 Site Surface and Textural Characteristics ............. .......................................................... 7 

Table 5 Threshold Dust-lofting Wind Speed in Various Desert Environments........................8 

Table 6 Cross-tabulation of Average Wind Speed and PM Concentrations...........................13 

Table 7 Cross-tabulation of Hourly Maximum Wind Gust and PM Concentration..................14

REFERENCES...........................................................................................................12
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1.0 Introduction 

 

Windblown dust has been and continues to be a pollutant of concern throughout the deserts of Arizona 

and southeastern California. The Arizona Department of Environmental Quality (ADEQ) requests that 

these dust episodes are excluded from regulatory decisions when the dust is associated with nonanthropogenic emissions or when control measures are overwhelmed by strong winds. During the last 

several years, these requests have increased substantially due to a greater deployment of continuous 

particulate matter (PM) monitors throughout Arizona. To supplement the exceptional event analyses for 

high wind and blowing dust events in Arizona, a review of historical meteorological and environmental 

data was conducted. Additionally, literature concerning the effects of strong winds on particulate 

emissions was examined. This literature review focused on (1) the theory behind dust suspension caused 

by wind; (2) laboratory experiments documenting wind speed thresholds that suspend soil particles; and 

(3) the frequency with which blowing dust occurs in the desert southwest as determined from 

meteorological data and observations of blowing dust. 

The Western Regional Air Partnership (WRAP) conducted a thorough review of literature relating to 

impacts of fugitive dust on air quality. Much of this information was compiled into WRAP’s Fugitive 

Dust Handbook (FDH) (WRAP FDH, 2006). While the handbook does not apply to ambient air quality 

monitoring, it does include specific information about various control measures for agricultural activity, 

construction activity, paved and unpaved roads, and more. Sections of this handbook cover the 

windblown suspension of dust from both anthropogenic and natural, barren areas. The FDH (2006) 

defines anthropogenic dust as “solids and dissolved solids entrained by wind passing over surfaces that 

have been disturbed or altered by humans beyond a natural range”. The handbook gives examples of such 

anthropogenic sources of dust including construction and mining sites, agricultural activities, material 

storage piles, landfills, vacant lots, roadways, and parking lots. The FDH defines natural dust as “solids 

and dissolved solids entrained by wind passing over surfaces that have not been disturbed or altered by 

humans beyond a natural range” (WRAP FDH, 2006). Examples of these natural dust sources include 

naturally dry riverbeds and lakebeds, barren lands, sand dunes, and non-agricultural grass, range, or forest 

lands. It is important to note that “natural and anthropogenic dust will often be indistinguishable and may 

occur simultaneously”, which means that while natural areas may emit dust during high wind events, 

those same areas will emit more dust when their surfaces have been or are in the process of being 

disturbed by human activities (WRAP FDH, 2006). One example of this would be a dry riverbed (a 

natural dust source) that has recently been the site of all-terrain vehicle activity (an anthropogenic 

influence). 

While wind generated emissions may result from open dust sources during high wind events, those 

emissions can exhibit a high degree of variability from one site to another, and may fluctuate widely at 

any given site. The FDH gives two groups of site characteristics that can cause such variability: 

(a) Properties of the exposed surface material from which the dust originates 

(b) Measures of energy expended by wind interacting with the erodible surface 

In addition to these site specific characteristics, the spatial and temporal variances in the interaction of the 

atmosphere with the surface also contribute to the variability of windblown dust emissions. 

2.0 Spatial and Temporal Variability of Windblown Dust Events 

Several studies have been conducted on the spatial and temporal variability of dust episodes associated 

with high wind events in Arizona and California. Dust events have been defined by a reduction in 

visibility to less than 11 km (Orgill and Sehmel, 1976; McTanish and Pitblado, 1987). Additionally, 

events with visibilities ≤ 1 km may have significant impacts on atmospheric pollution, health hazards, and 
2

environmental air quality (Jauregui, 1989a cited by Bach et al. 1996; Middleton, 1989a cited by Bach et 

al. 1996). Many studies have used these < 11 km and ≤1 km visibility criteria to assess the temporal and 

spatial variability of windblown dust events. From the literature reviewed, the one location that overlaps 

a majority of the studies is Yuma, AZ. Descriptive statistics of annual average dust events in Yuma range 

from 9.6 events per year to 27 events per year (Bach et al. 1996; Holcombe et al. 1996; Brazel and 

Nickling 1987; Brazel and Nickling 1984). Brazel and Nickling’s (1984) calculation of 9.6 events per 

year appears to be a low outlier in view of other studies. The disparity is likely due to differences in data 

and/or methodology and does not appear to be associated with the temporal coverage of the analyses. 

Brazel and Nickling’s (1984) results are likely an underestimate of blowing dust events in Arizona 

(personal communication, Brazel). When using the 11 km visibility criteria, most studies report annual 

dust episodes in Yuma to range from 22.4 to 27 events per year (Bach et al. 1996; Holcombe et al. 1996; 

Brazel and Nickling 1987). When using the 1 km visibility criteria, annual average dust episodes in 

Yuma range from 1.6 events per year to 4.7 events per year (Bach et al. 1996; Brazel and Nickling 1987; 

Brazel and Nickling 1984). The use of the different criteria indicates that the occurrence of dust episodes 

of varying severity in the desert southwest has been an ongoing phenomenon since observations have 

been taken. The more severe 1 km visibility criteria episodes likely make up a majority of the windblown 

dust PM10 exceedances in Yuma, AZ. While these events do not occur every day, it is not uncommon for 

blowing dust episodes to affect a single location several times over the course of any given year. 

 Table 2 – 

Frequency of Dust Episodes from Various Studies for Yuma, AZ

Annual Average Dust Episodes Source Time Period Vis < 11 km Vis < 1 km 

Bach et al (1996) 1973-1984 22.4 2.6 

Holcombe et al (1996) 1948-1978 27 N/A 

Brazel and Nickling (1987) 1942-1982 23.9 4.7 

Nickling and Brazel (1984) 1965-1980 9.4 1.6 

While much of the research on dust episodes in Arizona has focused on Yuma, AZ, Brazel and Nickling 

(1984) also analyzed data for other regions of Arizona (i.e., Winslow, Tucson, and Phoenix). Their 

analysis determined that dust episodes occur more frequently in Yuma and Phoenix in comparison to 

Tucson and Winslow. While Phoenix and Yuma experience blowing dust episodes on a yearly basis, the 

characteristics of these episodes can be quite different between the two locations. For example, while 

Yuma experiences more moderately intense dust storms (i.e. visibilities < 11 km but ≥ 1 km) in any given 

year, Phoenix experiences more intense dust storms (i.e. visibility ≤ 1 km). On the other hand, Phoenix 

and Yuma contain a similar distribution in dust episodes throughout a given year with peak occurrences in 

August and April respectively. Brazel and Nickling (1986) further analyzed these dust episodes and 

associated them with differing weather types. In general, dust episodes in Yuma are most often 

associated with synoptic weather patterns, specifically frontal passages. These episodes occur most often 

in the spring months when the mid latitude storm track reaches furthest south. Phoenix is affected most 

by dust episodes associated with thunderstorm outflow boundaries during the monsoon season in July and 

August. Phoenix is more often affected by short duration, high intensity dust storms due to its proximity 

to the Mogollon Rim and SE Arizona, where thunderstorms develop more frequently. Typical storm 

motion during the monsoon season is from east to west, but storms can also approach from the north and 

south. Essentially, storms can approach from an area starting at 0° (north) and rotating clockwise to the 

180° (south) azimuth. This is illustrated well in Brazel and Nickling’s (1986) Figure 9(c). It should be 

noted, however, that dust episodes in Yuma are also associated with thunderstorm outflow boundaries 

during the monsoon season. Similarly, Phoenix is also affected by non-monsoonal dust storms associated 

with frontal passages. While dust episodes caused by high winds are most common in the spring and 
3

summer for Yuma and Phoenix, respectively, these dust events can occur throughout the year for either 

location. 

Dust storms associated with different weather types will also have differing characteristics based on the 

weather type. For example, dust storms associated with synoptic scale weather types (i.e. frontal passages 

or trough passages) typically are longer in duration (on the order of several hours to 10 hours) and less 

intense (i.e. visibilities between 2 and 11 km). Dust storms that are associated with mesoscale weather 

types (i.e. thunderstorm outflow boundaries) tend to be shorter in duration (1 hour to several hours) but 

are more intense (visibilities between 1⁄4 mile and 5 miles). While each hour of a synoptic scale driven 

dust storm may be less intense than an hour in a mesoscale driven dust storm, the effect on human health 

when averaged over the entire day may be very similar (i.e. 24-hr average PM10 concentrations may be 

similar for the two types of dust storms). It should be noted that dust storms produced by synoptic type 

weather patterns are deemed synoptic because the area of high winds is typically fairly large. In many 

cases, however, the area of blowing dust may be much smaller than the extent of high winds due to the 

variability in soil characteristics throughout the region or the influences of local topography which can 

enhance wind speeds at a particular location. This is most often seen in areas such as El Centro, CA and 

Imperial, CA, and to a lesser degree, Yuma, AZ. 

The susceptibility to blowing dust varies greatly over the Sonoran Desert region of the southwestern 

United States. Holcombe, Ley, and Gillette (1996) determined that mountains, irrigated agricultural 

areas, wetlands, rivers and lakes, and urban areas all can be characterized as having a low susceptibility to 

blowing dust. Meanwhile, basin areas have susceptibilities ranging from low to moderate while dry 

riverbeds and lakebeds and non-irrigated portions of the Colorado River delta are highly susceptible to 

blowing dust, especially in areas where desert pavement is disturbed exposing fine-grained sediments 

from beneath (Holcombe et al., 1996). 

3.0 Threshold Friction Velocities and Threshold Wind Speeds 

Another factor that has received some attention in the literature is the dependence of soil type 

characteristics on particle movement by wind (e.g. Nickling, 1988; Gillette et al., 1980; Gillette et al., 

1982; Gillette, 1988; Nickling and Gillies, 1989; WRAP, 2006). There are three ways by which wind can 

transport soil particles: saltation, surface creep and suspension (Lyles 1988; Nickling 1988). Saltation 

normally occurs with particles that are too large to be suspended for a long period of time in the 

atmosphere. The process of saltation involves particles ranging from about 75 to 500 μm in diameter that 

are lifted from the surface and bounce within a layer close to the boundary between the air and the 

surface. Particles that are transported by surface creep can range from about 500 to 1,000 μm in diameter 

and move very close to the ground due to their larger size. Both wind friction and saltating particles can 

provide the mechanism for particles to move by surface creep (WRAP FDH, 2006). 

Generally, particles less than about 75 μm in diameter may be suspended in the atmosphere, and these 

particles tend to follow air currents given strong enough surface winds. The threshold wind speed for the 

onset of saltation, which helps cause wind erosion leading to windblown dust emissions, is dependant on 

soil texture and particle size (Nickling, 1988). Thus, smaller dimension particles tend to have the lowest 

threshold friction velocities and threshold wind speeds at 10m above the surface. The saltation process 

causes particles to collide resulting in the release of particles in the PM10 size range that are typically 

bound by surface forces to larger clusters of sediment (WRAP FDH, 2006; Lyles, 1988). 

WRAP developed a wind blown dust model in addition to creating the FDH. The model was based upon 

the characterization of various land types and soil conditions. To create this model, a number of 

assumptions were made, including a threshold wind velocity of 20 mph that is independent of land use 

and soil texture. This threshold wind speed assumption was based upon previous research measuring 
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both surface and ten meter threshold friction velocities and threshold wind speeds (WRAP, 2006; Lyles 

1988). In order to calculate friction velocities or threshold wind speeds the Prandtl Equation is used: 

 ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

o

z

z

z

u

u

κ

1

*

 Equation (1) 

Where uz is the wind velocity at height z (ms ̄1), u* is the friction velocity (ms ̄1), к is von Karman’s 

constant (~0.4), and zo is the aerodynamic roughness height (m) (Nickling and Gillies, 1989). 

Information about a surface’s aerodynamic roughness is needed in order to apply the relationship shown 

in Equation 1 and determine the threshold friction velocity of a surface or the corresponding 10m wind 

speed. Unfortunately, this roughness height information is not generally available or reported in land use 

databases. It is possible, however, to use the land use designation of varying surfaces to determine 

estimated aerodynamic roughness lengths based on previously reported values for the same or similar 

surfaces. Factors such as the presence of vegetation on a surface, as well as the height and period of 

cover of the vegetation, can introduce a degree of uncertainty to assigning an aerodynamic roughness 

length to a surface (WRAP, 2006). 

A number of studies have been conducted in an attempt to determine what threshold friction velocity is 

necessary to loft and transport dust from various soil types and under differing soil conditions. The 

WRAP (2006) Wind Blown Dust model study compiled threshold friction velocities that had been 

measured and estimated by some of this previous work. Table 2 shows one such compilation of data that 

compares threshold friction velocities for typical desert surfaces for two different surface conditions. The 

“Undisturbed” column presents surface threshold friction velocities for the natural state of each surface 

while the “Disturbed” column provides the threshold friction velocities for each surface that has 

undergone significant anthropogenic disturbance. In each case, the threshold friction velocity for 

disturbed soil is lowered significantly from the threshold friction velocity for undisturbed soil (WRAP, 

2006). The effect that anthropogenic disturbances have on the threshold friction velocity at each site type 

is significant and ranges from a low of a 17% decrease for the alluvial fan type soils to a 92% decrease for 

the prairie site type. The average percent change in threshold friction velocity when comparing 

undisturbed surfaces to disturbed surfaces across all site types is a 55% drop from the undisturbed value 

(WRAP, 2006). 

Sources: Gillette et al. (1980, 1982), Gillette (1988), and Nickling and Gillies (1989), WRAP (2006). 

Table 2 – 

Threshold Friction Velocities for Typical Desert Surface Types

Average 

t u* (ms ̄1) 

Average 

t u* Site Type (ms ̄1) 

Undisturbed 

Number of 

Data Points 

Disturbed 

Number of 

Data Points % Change 

Agricultural Fields 1.29 41 0.55 37 57 

Alluvial Fans 0.72 2 0.60 2 17 

Desert Flat 0.75 4 0.51 4 32 

Desert Pavement 2.17 4 0.59 5 73 

Fan Surface 1.43 5 0.47 5 67 

Play, Crusted 2.13 4 0.63 15 70 

Playa 1.46 12 0.58 25 60 

Prairie 2.90 1 0.24 3 92 

Sand Dune 0.44 4 0.32 4 27 
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Nickling and Gillies (1989) used a portable wind tunnel at thirteen study sites, twelve of which were 

located in Arizona with one in Algodones, California, in order to analyze threshold friction velocities at 

sites exhibiting an array of different characteristics. Threshold friction velocities were determined by 

slowly raising the wind velocity in the portable wind tunnel until movement of particles was noted by 

observers. This process was repeated multiple times at each site to produce a reliable dataset. Resultant 

velocities were found to vary significantly across the 13 sampled surfaces. Surface threshold friction 

velocities ranged from 17.2 cms ̄1 to 58.2 cms ̄1. These threshold friction velocities were then used to 

calculate the equivalent 10m threshold friction velocities. The threshold wind velocities at the 10m level 

varied from lows of 5.11 ms ̄1 (11.43 mph) at a mine tailings site in Hayden, AZ and 6.68 ms ̄1 (14.94 

mph) in the dry Salt River channel in Mesa, AZ to highs of 16.59 ms ̄1 (37.11 mph) at a Yuma, AZ 

agricultural site and 18.31 ms ̄1 (40.96 mph) at the dune flats of Algodones, CA (Nickling and Gillies, 

1989). Full descriptions of the test sites and results of the wind tunnel studies conducted by Nickling and 

Gillies (1989) are included in Table 3 and Table 4 below.

 

Table 3 – 

Aerosol Emission Test Results 

Site 

u*

(m/sec) 

u10m

(m/sec) zo (cm)

Mean 

Grain Size 

(μm) 

% Silt 

& Clay 

% 

Aggregates > 

.84 mm 

Mesa, AZ 

(Agricultural) 0.569 15.63 0.0331 1077.0 18.6 26.8 

Maricopa, AZ 

(Agricultural) 0.578 13.82 0.1255 749.3 11.2 37.3 

Yuma, AZ 

(Agricultural) 0.582 16.59 0.0224 642.6 8.8 18.5 

Casa Grande, AZ 

(Abandoned Agricultural) 0.246 7.80 0.0067 234.4 26.6 2.4 

Glendale, AZ 

(Construction Site) 0.530 14.69 0.0301 1275.1 24.7 27.9 

Tucson, AZ 

(Construction Site) 0.251 7.26 0.0191 1010.9 14.3 21.0 

Ajo, AZ 

(Mine Tailings) 0.228 6.68 0.0176 335.3 8.9 0.5 

Hayden, AZ 

(Mine Tailings) 0.172 5.11 0.0141 271.8 27.3 3.0 

Algodones, CA 

(Dune Flats) 0.625 18.31 0.0166 2794.0 15.2 60.3 

Yuma, AZ 

(Scrub Desert) 0.386 11.33 0.0163 1038.8 17.2 22.4 

Yuma, AZ 

(Disturbed Scrub) 0.320 8.11 0.0731 591.8 3.2 7.7 

Tucson, AZ 

(Santa Cruz River Terrace) 0.180 7.26 0.0204 1950.7 20.9 37.4 

Mesa, AZ 

(Salt River Channel) 0.218 6.68 0.0100 398.7 27.7 11.3 

Source: Nickling and Gillies, 1989. 

It is interesting to note that of the four agricultural sites tested, the abandoned site in Casa Grande had the 

lowest threshold friction velocity (Table 3). Based upon the site descriptions, this may be a result of the 

abandoned field being relatively smooth with a weakly crusted surface. In addition, the abandoned field 
6

appears to have lacked the large clods covering its surface that the other three active agricultural fields 

possessed. Equally interesting is that the sand dune flats of Algodones, California had the highest average 

threshold friction velocity and 10m wind speed (Table 3). The authors note a relation between aggregate 

grain size distribution and threshold velocities, and this may explain why the dune flats threshold velocity 

was found to be so high. The dune flats site had the highest percentage of aggregate particles greater than 

0.84mm in size of all the test sites. Interestingly, previous research has suggested that the Algodones 

Dunes are likely the source for blowing dust that is commonly reported in Yuma, AZ (Brazel and 

Nickling 1986). Holcombe et al. (1996), on the other hand, hypothesized that the source for observational 

reports of blowing dust in Yuma may be the East Mesa area instead. All studies show that observational 

reports of blowing dust in Yuma were associated with wind speeds substantially lower than the 

Algodones 10m threshold velocity reported by Nickling and Gillies (1989). With the source areas in such 

close proximity to one another, it becomes difficult to determine which source area is responsible for 

blowing dust in Yuma, AZ. Other potential explanations for the above results may lie in the soil moisture 

content of each site, which would tend to vary both by site and by time of year. Neither of these 

important pieces of information was examined nor reported in the Nickling & Gillies (1989) study. 

Upon first glance, the threshold velocity results from Nickling & Gillies (1989) might lead one to believe 

that wind erosion could be initiated at many of the sites under normally occurring winds, but a number of 

relatively high threshold values at three of the agricultural sites as well as test sites in Mesa, Maricopa, 

Yuma, AZ and a construction area in Glendale, AZ suggest that a major wind blown dust event would be 

a relatively infrequent occurrence. It is unclear from the study as to what extent the higher threshold 

values at these sites can be attributed to any specific control measures that may have been in place. 

Additionally, natural factors such as recent precipitation, mean grain size, or soil type may have played a 

role in causing the higher threshold values. 
7

Table 4 – 

Site Surface and Textural Characteristics 

Site Site Characteristics 

Mesa, AZ 

(Agricultural) 

Flat, laser leveled for irrigation; no vegetation or surface trash; double 

disked 2 days prior to testing; large clods of varying size covering the 

surface. 

Maricopa, AZ 

(Agricultural) 

Flat, laser leveled for irrigation; no vegetation or surface trash; secondary 

tillage operations in preparation for cotton planting; large clods of 

varying size covered the surface. 

Yuma, AZ 

(Agricultural) 

Flat, laser leveled for irrigation; no vegetation or surface trash; surface 

was loose and friable with few large clods. 

Casa Grande, AZ 

(Abandoned Agricultural) 

Flat, smooth, laser leveled at some previous time; sparse vegetation; 

primarily annual grasses; weakly crusted surface; easily disturbed. 

Glendale, AZ 

(Construction Site) 

Flat, smooth, machine-leveled surface; no vegetation; surface soil heavily 

pulverized by earthmoving machinery and regularly watered for dust 

control; easily disturbed by vehicular traffic. 

Tucson, AZ 

(Construction Site) 

Very flat, machine-leveled surface; no vegetation; surface soil heavily 

pulverized by earthmoving; fetch length over 1 km in all directions; 

regularly watered for dust control. 

Ajo, AZ 

(Mine Tailings) 

Extremely flat, extensive copper mine tailings; no vegetation; mainly 

unconsolidated sediment with some sparsely intermixed crusted areas; 

fetch lengths greater than 1 km. 

Hayden, AZ 

(Mine Tailings) 

Extremely flat copper tailings; no vegetation; light crust is easily 

disturbed; fetch lengths greater than 1 km. 

Algodones, CA 

(Dune Flats) 

Relatively flat, extensive outwash deposits adjacent to Algodones Dune 

complex; sparse vegetation cover; surface weakly crusted; lag of fine 

pebbles; heavily disturbed by off-road vehicles. 

Yuma, AZ 

(Scrub Desert) 

Relatively flat; sparse vegetation cover; surface is weakly crusted; lag of 

fine pebbles; heavily disturbed by off-road vehicles. 

Yuma, AZ 

(Disturbed Scrub) 

Slightly undulating surface; sparse vegetation cover; primarily annual 

grasses, sagebrush and creosote bush; surface soil is very loose with little 

cohesive structure; heavily disturbed by off-road vehicles. 

Tucson, AZ 

(Santa Cruz River Terrace) 

Flat terrace deposits adjacent to river; sparse vegetation cover; high silt 

content with fin gravel lag deposit. 

Mesa, AZ 

(Salt River Channel) 

Slightly undulating river bed; no vegetation; surface is very loose with no 

evidence of crusting; heavily disturbed by off-road vehicles. 

Source: Nickling and Gillies, 1989. 

Nickling and Gillies (1989) stresses an increased potential for aerosol (lofted dust) production at those 

sites which experience a periodic disturbance of soil such as the river channels and mine tailing sites, as 

they tended to exhibit lower threshold friction velocities. This was also a point of emphasis in the WRAP 

(2006) Wind Blown Dust model work that compared threshold friction velocities for disturbed and 

undisturbed soil of various study sites by compiling available data from previous research such as that 

done by Nickling and Gillies. Precipitation is another factor to consider, as periodic rains help to build up 

and maintain a surface crust on the soil by redistributing clay particles on the surface, creating a seal as 

they are left behind after infiltration and evaporation processes occur (Nickling and Gillies (1989). It is 

also possible that severe precipitation in the form of brief heavy rain or hail may serve to break up an 

already developed surface crust and naturally disturb the soil, leading to a greater possibility of a 

windblown dust event. It has also been shown that individual sites can have vastly different soil 

conditions based upon any soil vegetation, moisture, or the overall proportion of larger sized grains. That 
8

is to say that not all agricultural fields, construction sites, and sand dune areas will experience the same 

threshold friction velocities as all other agricultural fields, construction sites, and sand dunes. 

The University Corporation for Atmospheric Research (UCAR) maintains an educational website at 

http://www.meted.ucar.edu/mesoprim/dust that provides a comprehensive overview of the forecasting of 

dust storms while including a detailed explanation of the physical processes involved in the wind 

transport of sediment. It gives 15 knots (~17 mph) as a rule of thumb as to the minimum wind speed that 

is necessary for mobilizing dust without regard to varying soil types. After winds decrease below the 

initiation levels dust storms can continue and maintain the same intensity due to the breakdown in the 

bonds between dust particles and the surface. Once this bond is broken the saltation process can continue 

to allow dust to lift (UCAR, 2003). While 17 mph is given as an approximate baseline minimum for 

generating wind-blown dust, more detailed breakdowns of threshold wind speeds for various desert 

environments are also given in the UCAR educational website for the forecasting of dust storms (Table 

5). These threshold wind speeds range from near 15 mph for fine to medium sand dune areas up to 40 

mph in desert areas where desert pavement has been well-developed. “Desert pavement” is the term 

given for the thin veneer of rocks that is left behind once wind and other erosive processes remove finer 

grained material from the desert surface leaving only larger grains and stones on the surface. The 

formation of desert pavement thus suppresses windblown dust as fewer finer grained particles are 

available on the surface for transport 

There is a large difference between the threshold wind speed given for sand dune areas on the UCAR 

webpage and that given in the Nickling and Gillies (1989) study. As previously mentioned, particle size 

(and possibly moisture content) provides a possible explanation for this disparity in threshold wind 

speeds. Additionally, the Nickling and Gillies (1989) study looked at a site on an outwash area adjacent 

to a sand dune complex composed of a significant percentage of larger sized grains whereas the UCAR 

page describes the sand grains of the dune area they studied as being fine to medium sized. This may 

account for the greater than twenty mph disparity in threshold wind velocities given for sand dune areas 

between the two studies. In Arizona, the most common source environments for windblown dust are 

likely to range from varying degrees of desert pavement to desert flats and salt flats, but dune areas 

outside of the state both to the south and west can also serve as potential particulate sources, particularly 

for Yuma, AZ. The minimum wind speeds necessary for dust transport in these types of source 

environments, according to the bulk of the research, has been shown to range from approximately 17 mph 

to 35 mph.

Table 5 – 

Threshold Dust-lofting Wind Speed in Various Desert Environments 

Environment Threshold 

Wind Speed 

Fine to medium sand in dune-covered areas 10-15 mph 

Sandy areas with poorly developed desert pavement 20 mph 

Fine material, desert flats 20-25 mph 

Alluvial fans and crusted salt flats (dry lake beds) 30-35 mph 

Well-developed desert pavement 40 mph 

Source: UCAR, 2003. 

Research by Holcombe et al. (1996) focuses on the effect that both prior precipitation and particulate 

source area characteristics can have on threshold friction velocities for wind blown dust events. In their 

study, they mention that “threshold friction velocities computed from mean hourly wind speeds (MHWS) 

recorded during dust events are significantly lower than those obtained from wind tunnel experiments 

over loose desert soils” (Holcombe et al., 1996). However, the authors determine that when threshold 

friction velocities are computed from mean extreme wind speeds (maximum gusts) they are comparable 
9

to resultant threshold wind speeds obtained from wind tunnel experiments. That is to say that the friction 

velocities computed from mean extreme winds, i.e., maximum wind gusts, are more in line with wind 

tunnel studies than are threshold friction velocities computed from MHWS, and thus maximum wind gust 

data may be a better predictor of windblown dust (or at least the threshold friction velocities that are 

required to begin the saltation process which can lead to dust emissions) than are mean hourly wind 

speeds (Holcombe et al., 1996). 

4.0 Best Available Control Measures (BACM) Effectiveness 

WRAP’s Fugitive Dust Handbook (FDH, 2006) contains a plethora of information relating to the 

numerous strategies available to control dust emissions. The FDH goes into great detail in describing the 

different control measures and breaks them down into several source categories. Common control 

strategies for wind erosion include: 

• Planting trees or shrubs as a windbreak 

• Create cross-wind ridges 

• Erect artificial wind barriers 

• Apply dust suppressant or gravel 

• Revegetate; apply cover crop 

• Water exposed area before high winds 

 

These control measures have differing reported control efficiencies ranging from 25% (planting trees or 

shrubs) to 90% (revegetating and watering exposed areas before high winds). In addition to the control 

efficiencies reported in the FDH, two field studies were performed for the South Coast Air Quality 

Management District (Fitz, 1996; MRI, 1995) that specifically mention wind speeds at which BACM 

becomes overwhelmed. Fitz (1996) concluded that BACM appeared to be effective at reducing PM10

emissions at wind speeds up to 18 m/s (about 40 mph); however, the small sample size in this study 

makes it difficult for these results to be conclusive. Additionally, this study was specific to emissions 

generated from a local landfill in California. It is not clear how factors mentioned in sections 2.0,3.0, and 

5.0 would affect this defined wind speed. Information from Tables 3 and 4 (specifically data and 

metadata from the Tucson and Glendale construction sites) may suggest that wind speeds at which 

BACM becomes overwhelmed would also be highly variable from one site to another. In another study 

on the effective of BACM, the Midwest Research Institute (MRI, 1995) simply concluded that even moist 

soils became erodible at higher wind speeds. The emissions from moist soils became similar to that of 

dry soils when the moist soils were subjected to wind speeds 15-20 mph higher than winds impinging on 

dry soil. However, by looking at the data tables from this study, it is difficult to determine the different 

threshold velocities for moist and dry soils. With the limited research that has been conducted on this 

specific topic and the lack of detail in the few studies that have been performed, it does not seem 

appropriate to define a critical wind speed in which BACM becomes overwhelmed. Furthermore, such a 

critical wind speed would likely differ for each soil type and/or land use type. A more detailed review of 

general BACM effectiveness and control efficiencies can be found in “High Wind Exceptional Events and 

Control Measures for PM10 Areas”. 

5.0 Other Influential Factors in the Formation and Impact of Windblown Dust 

Data from Holcombe et al. (1996) suggest that a wide range of wind speeds is capable of producing 

blowing and/or suspended dust in the Yuma area. Of the dust episodes in Yuma, 95% contain hourly 

mean wind speeds that range from 2.9 m/s to 12.96 m/s. This compares to a range of 3.8 m/s to 15.7 m/s 

at Blythe, indicating that dust may become suspended more easily in the Yuma area compared to Blythe. 

This may be a factor of Yuma receiving slightly less precipitation and/or various differences in soil 
10

characteristics and available dust sources between Yuma and Blythe. In an attempt to determine the 

effect of prior precipitation on blowing dust episodes in Yuma, MacKinnon et al. (1990) used Nickling’s 

(1988) wind tunnel data to calculate a 10 meter threshold wind speed of 9 m/s, which is similar to the 

median wind speed obtained from observational reports in the Holcombe et al. (1996) study. In general, 

dust episodes in Yuma are most common with 10 meter wind speeds between 5 m/s and 13 m/s with a 

median of 9 m/s. The large range in wind speeds is likely due to the variability in precipitation, 

prevailing wind direction, vegetation, anthropogenic sources, and other factors. 

Bach et al. (1987) reported that annual precipitation explains about 20% of the variance in the incidence 

of blowing dust. When antecedent precipitation (previous winter’s rainfall) is taken into consideration the 

explained variance increases to 26.5%. When taking the winter precipitation from previous 2 years into 

account, the explained variance increases to 42.3%. This effect of antecedent precipitation likely also 

includes a vegetative coverage factor with it. The general rule appears to be that the more rainfall a 

region receives the more vegetation grows and the greater its ability to keep soils from becoming 

suspended by high winds. Holcombe et al. (1996) concludes that precipitation can be an important factor 

in limiting soil suspension. They do, however, note that in places like Yuma where threshold MHWSs lie 

well below MHWSs recorded during most dust events, local mean annual precipitation could likely 

increase substantially without significantly reducing the occurrence of blowing dust episodes. While 

prior precipitation events do help explain some of the variance in dust events, there is a fair amount of 

variance that is still not explained by precipitation. This variance can likely be attributed to other factors 

such as soil type, prevailing wind direction, anthropogenic activity, control measures, and changes in land 

use / land cover. 

While wind speed is obviously important for dust suspension, a number of articles also point out that the 

directional component of the wind is also important as it is an indicator of potential dust sources 

(MacKinnon et al. 1990; Holcombe et al. 1996; Brazel and Nickling 1986). The Holcombe et al. (1996) 

paper is particularly interesting as it attempts to apply a vulnerability index to each of the 16 directions. 

This type of analysis seems to be fairly effective at determining potential dust sources. Several articles 

indicate that for Yuma, the west and west-northwest directions are associated with high vulnerability and 

high frequency, while high winds from the east contain a high vulnerability but a low frequency. 

Conversely, high winds from the south-southeast and northerly directions frequently occur, but have a 

low vulnerability. The differences in vulnerability in relation to wind direction are an indication that soil 

characteristics may be an important factor in dust episodes. While not explicitly stated in the study, one 

could even define differing threshold friction velocities for each direction. 

6.0 The Effect of Particle Size and Turbulence on Dust Settling Rates 

Thus far the literature has suggested that the meteorological phenomena required to produce dust events 

in the desert southwest are highly variable. In fact, factors such as wind speed, wind direction, antecedent 

precipitation, vegetation coverage, anthropogenic activity, and the location of dust sources all may 

potentially contribute to the variability in blowing dust episodes. The UCAR dust forecasting education 

website discusses dispersion and settling rates of lofted dust in relation to buoyancy turbulence (UCAR, 

2003). While such turbulence can work to help disperse an existing dust plume, it can also act to keep 

dust particles in suspension for a longer period of time. Given extremely turbulent flow, it is possible for 

dust to be lofted to heights well above 3000m (10000 ft.). As long as the upward lift acting on a particle 

is greater than that particles terminal velocity, the particle should remain aloft. The terminal velocity of a 

particle is directly proportional to its size, and as gravity acts to accelerate a dust particle downwards, 

atmospheric resistance acts against it as it falls. Acceleration continues only until the particle reaches its 

terminal velocity, and from that point on, the particle falls at a constant rate so long as no other factors 

such as atmospheric turbulence or precipitation influence it (Bagnold, 1984 cited by Wilkerson, 1991). 

UCAR (2003) states that “without sustained turbulence, dust generally settles at a rate of 1000 feet per 
11

hour”. This means that if dust is lofted to 5000 ft in extremely turbulent flow (i.e. downdrafts from 

thunderstorms), it may take the at least 5 hours to settle once turbulent winds decrease to below terminal 

velocity (i.e., light and variable winds). This can result in dust, once initially suspended by turbulent 

flow, remaining elevated in the atmosphere and being transported by light winds to impact areas that do 

not experience the extremely turbulent winds that initialized the dust event. This scenario most often 

occurs during the monsoon season in July and August when turbulent flow produced by thunderstorms is 

highly variable across space and time. While dust episodes driven by synoptic scale weather types would 

likely have similar wind speeds at the source area compared to the point where wind measurements are 

taken, this assumption may not apply to dust produced from mesoscale storms as the wind speed at the 

source area may be substantially different than the wind speed measured at the observation point 

(MacKinnon et al. 1990). The 1000 ft/hr settling rate has been reported for particles between the sizes of 

10 to 50 micrometers; however this rate is highly dependent upon various environmental conditions 

(Larson, 1971 cited in Wilkerson, 1991). These dust settling rate factors include turbulence, humidity, 

and precipitation. Other studies have reported settling rates ranging from over 10000 ft/hr for particles 

greater than 150 microns to 36 ft/hr for particles as small as 5 microns (Greveris, 1977 cited in Wilkerson, 

1991). Moisture is especially important in limiting the ability of dust to remain lofted in the atmosphere 

given the hygroscopic nature of most dust particles. 

7.0 Literature Conclusion 

In reviewing the literature, most studies have focused on the meteorological conditions that produce 

blowing dust. Specifically, wind speed is the most important factor; however, wind direction and 

antecedent precipitation also contribute to characteristics of dust emissions. There is also speculation that 

other factors such as vegetation and soil types affect dust emissions as well by raising threshold friction 

velocities and thus threshold wind speeds at 10m above the surface. While this information is important, 

for exceptional event purposes one must be able to show that a dust event is ‘natural and unavoidable’ or 

at least that there was an attempt to control dust emissions. For the ‘natural and unavoidable’ case, the 

prior studies contain some very useful information, particularly in the case of Yuma, AZ where the 

Algodones dunes (a natural dust source) are the suspected source of dust emissions. However, within an 

urban setting where Best Available Control Measures (BACM) are required to suppress dust emissions, it 

no longer becomes important to simply determine the wind speed at which dust emissions occur, but 

instead, the speed at which BACM is overwhelmed. While there is some literature on the effectiveness of 

BACM (i.e. WRAP Fugitive Dust Handbook, 2006), there appears to be very few studies that relate this 

effectiveness to wind speeds. With several factors influencing threshold friction velocities, defining exact 

wind speeds for overwhelming BACM or even dust suspension in general becomes a complicated 

process. Perhaps, the results from this literature review are best summarized by Nickling’s (1988) 

conclusion which states, “The observations of this study suggest that the threshold of most natural 

sediments should not be defined by a finite value but by a range of threshold shear velocities.” 

8.0 EXAMINATION OF HISTORICAL WIND & PARTICULATE MATTER DATA IN 

 ARIZONA - WIND THRESHOLD EFFECTS 

Part of the justification for flagging an elevated particulate matter event associated with blowing dust is to 

show the winds were “unusual.” This term is used in the preamble of the Exceptional Events Rule. The 

literature review provided significant justification for what constitutes unusual from various contexts. In 

order to place perspective on the issue for monitors in Arizona, an analysis of wind and particulate matter 

data is needed. 

An examination of the measurements from several monitors where hourly measurements of the average 

wind speed, the maximum wind gust during the hour, and the average PM10 concentration from TEOM 
12

monitors was performed. Data from April 1, 2005, through August 31, 2009, for four monitors operated 

by the Maricopa County Air Quality Department (MCAQD) over a broad geographic region were used in 

the analysis. These monitors were chosen because of the availability of a very long continuous record of 

data (38,000+ hours) for each monitor. The monitors included a monitor to the far west of the Phoenix 

area (Buckeye), two monitors in the central basin near the Salt River (West Forty Third Avenue and 

Durango Complex), and one monitor in the far east Valley (Higley). All of these monitors have 

experienced high ambient concentrations of particulate matter. 

The monitoring data were extracted from the ADEQ Air Assessment Ambient Database (AAAD) which 

archives these readings on a daily basis. The winds speeds were rounded to the nearest 1 mile per hour, 

and the ambient PM10 concentrations were rounded to the nearest 1 microgram per cubic meter (μg/m3

). 

The ambient concentration data tend to be log-normally distributed. It was decided to create a simple 

cross-tabulation of the raw wind speed and particulate matter data for each monitor. In order to develop a 

useful consolidation of the data, wind and particulate matter values were transformed into categories. The 

following transformations were used to compute the categories: 

 For Wind: WindCat = 10.0*Log10(Speed_mph + 1.0) (rounded to the nearest whole number) 

 For PM10: PMCat = 5.0*Log10(PM_ug/m3 + 1.0) (rounded to the nearest whole number) 

The choice of the scaling was to generate approximately 17 categories over the normal range of values 

found in the database. 

In examining the cross-tabulated information, one would expect to observe two phenomena. For monitors 

in the central part of the valley, there could be a slight “stagnation” effect observed in the data. This 

would be where the mode of the distribution for a given wind speed category would go down as the wind 

speed increased from very low levels to moderate levels. Also, for all monitors, there should be a 

threshold effect, where the mode of the particulate matter concentration data for a given wind speed 

category should increase as the wind speed category increases. Between these two extremes, there would 

be little correlation between wind speed and particulate matter concentration. 

Table 6 presents the cross-tabulated information for hourly average wind speed versus the particulate 

matter concentrations for the four monitors. The orange/pink highlighting in the figure is where the 

median of the distribution occurs for each wind speed category. The yellow highlighting shows the range 

where 80% of the data reside. The green highlighting shows the frequency of the higher wind speeds. It 

is noted that these represent approximately the highest 5% of the wind data (approximately 2,000 out of 

40,000 observations). All four monitors demonstrate a strong “threshold” effect for wind speed when 

hourly average wind speeds exceed 10 miles per hour. Also, the West Forty Third Avenue and Durango 

Complex monitors appear to have a slight “stagnation” effect compared to the Buckeye and Higley 

monitors. 

Table 7 presents the same cross-tabulated information as Table 6, except that instead of using average 

wind speeds, maximum hourly wind gusts are used to display the relationship between wind and PM10

concentrations. The same highlighting scheme used in Table 6 is also applied to Table 7. All four 

monitors demonstrate a strong “threshold” effect for wind speed when hourly wind gusts exceed 20 miles 

per hour. 

It should be noted that there is not a “bright-line” in this type of data, only statistical tendencies. During 

some events, relatively low local wind speeds could be associated with very high particulate matter 

concentrations if dust is transported over a long distance. This is typical of a haboob, where local winds 

in the area of the storm may exceed 70 miles per hour, and create a wall of dust. The wind speed 

diminishes during transport but the suspended dust can be delivered over very long distances. 
STATION

 PM10 Category < 1 1 mph 2 mph 3 mph 4 mph 5-6 7 mph 8-10 11-13 14-16 17-21 22-27 28-34 TOTAL 

1995-3161 ug/m3 - - - - - - - - 1 - 1 2 - 4 

1258-1994 ug/m3 - - 1 - - 1 - 3 - - 1 - - 6 

794-1257 ug/m3 - - 3 4 3 - 1 - - 1 2 5 - 19 

501-793 ug/m3 1 5 6 6 3 5 1 5 14 10 13 6 - 75 

316-500 ug/m3 5 19 11 9 6 6 4 17 26 19 28 3 1 154 

199-315 ug/m3 24 43 40 31 27 32 7 22 30 48 42 8 1 355 

125-198 ug/m3 65 206 186 167 128 149 48 89 65 87 37 9 - 1,236 

79-124 ug/m3 228 522 607 520 415 575 171 296 170 77 53 4 - 3,638 

50-78 ug/m3 436 998 1,183 1,076 859 1,193 401 741 260 94 27 3 - 7,271 

31-49 ug/m3 438 1,218 1,546 1,438 1,282 1,852 605 1,091 330 80 21 - - 9,901 

19-30 ug/m3 298 942 1,220 1,253 1,136 1,735 599 956 254 54 12 1 - 8,460 

12-18 ug/m3 122 505 603 679 552 1,091 350 544 121 23 4 2 - 4,596 

7-11 ug/m3 58 189 258 258 267 397 133 217 64 16 2 - - 1,859 

5-6 ug/m3 7 40 53 45 46 86 34 56 24 8 - 1 - 400 

3-4 ug/m3 9 21 27 29 29 54 19 31 6 7 1 - - 233 

1-2 ug/m3 1 12 16 11 16 34 9 20 8 1 1 - - 129 

<1 ug/m3 1 7 13 12 8 20 16 13 8 2 2 - - 102 

TOTAL 1,693 4,727 5,773 5,538 4,777 7,230 2,398 4,101 1,381 527 247 44 2 38,438 

 PM10 Category < 1 1 mph 2 mph 3 mph 4 mph 5-6 7 mph 8-10 11-13 14-16 17-21 22-27 28-34 TOTAL 

1995-3161 ug/m3 - - - - - - - - 1 - - - 1 

1258-1994 ug/m3 - - - - - - 1 1 - 1 4 2 9 

794-1257 ug/m3 - 1 2 4 1 - 1 1 1 5 21 5 42 

501-793 ug/m3 2 13 6 1 - 4 1 6 7 22 38 4 104 

316-500 ug/m3 20 101 74 25 9 9 4 10 24 50 28 2 356 

199-315 ug/m3 68 380 349 153 54 39 9 32 70 72 22 1 1,249 

125-198 ug/m3 106 794 883 470 165 169 29 88 158 98 21 1 2,982 

79-124 ug/m3 193 1,384 1,547 850 403 438 112 288 235 72 6 - 5,528 

50-78 ug/m3 255 1,515 1,754 1,274 828 847 272 634 274 48 1 - 7,702 

31-49 ug/m3 192 1,333 1,791 1,383 1,165 1,519 441 894 232 28 1 - 8,979 

19-30 ug/m3 114 777 1,080 1,120 906 1,367 432 655 109 16 - - 6,576 

12-18 ug/m3 49 362 454 492 451 567 194 271 47 11 - - 2,898 

7-11 ug/m3 11 134 183 190 161 235 65 120 25 4 2 - 1,130 

5-6 ug/m3 5 26 34 36 32 40 25 26 5 1 - - 230 

3-4 ug/m3 2 12 22 22 20 32 18 22 5 2 - - 157 

1-2 ug/m3 - 8 14 9 9 14 8 8 3 - - - 73 

<1 ug/m3 1 8 4 6 6 9 4 5 4 - - - 47 

TOTAL 1,018 6,848 8,197 6,035 4,210 5,289 1,616 3,061 1,200 430 144 15 38,063 

 PM10 Category < 1 1 mph 2 mph 3 mph 4 mph 5-6 7 mph 8-10 11-13 14-16 17-21 22-27 28-34 TOTAL 

1995-3161 ug/m3 - 1 - - - - - - 1 1 - - 3 

1258-1994 ug/m3 - 1 - - - - - - - - - - 1 

794-1257 ug/m3 - 2 3 1 - - 1 1 - 3 6 3 20 

501-793 ug/m3 2 7 4 - - 2 3 10 7 9 13 - 57 

316-500 ug/m3 36 67 46 13 3 5 3 14 26 19 10 1 243 

199-315 ug/m3 82 310 169 56 12 16 7 33 47 32 26 - 790 

125-198 ug/m3 187 932 526 198 89 66 19 85 67 65 14 - 2,248 

79-124 ug/m3 377 1,622 1,123 535 298 321 80 178 126 72 6 - 4,738 

50-78 ug/m3 314 1,942 1,724 1,034 686 916 284 535 238 38 - - 7,711 

31-49 ug/m3 310 1,713 1,866 1,451 1,142 1,615 542 948 247 21 - - 9,855 

19-30 ug/m3 142 927 1,288 1,150 1,033 1,510 515 785 151 9 - - 7,510 

12-18 ug/m3 44 354 531 545 488 707 228 317 57 3 - - 3,274 

7-11 ug/m3 16 162 203 204 197 258 86 115 16 5 - - 1,262 

5-6 ug/m3 2 21 50 41 40 60 25 27 8 1 - - 275 

3-4 ug/m3 1 18 24 33 19 36 18 29 3 1 - - 182 

1-2 ug/m3 - 11 9 18 11 22 8 11 4 - - - 94 

<1 ug/m3 - 11 14 12 7 11 - 17 1 - - - 73 

TOTAL 1,513 8,101 7,580 5,291 4,025 5,545 1,819 3,105 999 279 75 4 38,336 

 PM10 Category < 1 1 mph 2 mph 3 mph 4 mph 5-6 7 mph 8-10 11-13 14-16 17-21 22-27 28-34 TOTAL 

3162+ ug/m3 - - - - - - - - - - 1 - - 1 

1995-3161 ug/m3 - - - - - - - - 2 - 1 1 - 4 

1258-1994 ug/m3 - - - - - - - 1 - - 1 - - 2 

794-1257 ug/m3 - - 1 - 1 - - 1 2 2 5 2 1 15 

501-793 ug/m3 1 1 2 3 3 8 - 6 11 17 14 - - 66 

316-500 ug/m3 3 30 19 16 11 15 8 22 20 28 20 - - 192 

199-315 ug/m3 10 79 97 72 63 71 14 31 43 40 13 - - 533 

125-198 ug/m3 52 240 264 240 196 276 58 89 68 53 9 - - 1,545 

79-124 ug/m3 76 427 656 618 533 595 132 189 133 54 6 - - 3,419 

50-78 ug/m3 110 772 1,233 1,356 1,135 1,244 236 399 135 31 9 - - 6,660 

31-49 ug/m3 128 845 1,596 1,933 1,769 2,031 460 582 172 36 1 - - 9,553 

19-30 ug/m3 73 680 1,308 1,681 1,638 2,071 501 582 109 10 1 - - 8,654 

12-18 ug/m3 49 388 704 876 876 1,092 232 314 57 15 - - - 4,603 

7-11 ug/m3 24 190 322 351 353 432 124 160 27 9 1 - - 1,993 

5-6 ug/m3 4 40 54 68 70 107 31 55 10 2 - - - 441 

3-4 ug/m3 4 13 34 33 40 66 17 38 6 2 - - - 253 

1-2 ug/m3 3 4 14 23 12 30 10 16 2 - - - - 114 

<1 ug/m3 - 7 6 9 9 10 10 9 1 - - - - 61 

TOTAL 537 3,716 6,310 7,279 6,709 8,048 1,833 2,494 798 299 82 3 1 38,109 

HIGLEY

WIND SPEED CATEGORY (HOURLY AVERAGE WINDS - miles per hour)

BUCKEYE WEST 43RD AVE

Table 6. Crosstabulation of Average Wind Speed and PM Concentrations

DURANGO COMPLEX
STATION

 PM10 Category < 1 1 mph 2 mph 3 mph 4 mph 5-6 7 mph 8-10 11-13 14-16 17-21 22-27 28-34 35-43 44+ TOTAL

1995-3161 ug/m3 - - - - - - - - - - - - 1 - 1 2 

1258-1994 ug/m3 - - - - - - - 1 1 1 - 2 - - 1 6 

794-1257 ug/m3 - - - - - 3 - 7 - 1 - - - 3 5 19 

501-793 ug/m3 - - - 3 1 7 4 8 1 - 2 9 16 16 8 75 

316-500 ug/m3 - - 1 4 9 21 9 10 5 6 2 11 43 29 3 153 

199-315 ug/m3 1 2 3 15 26 68 23 35 13 9 16 36 60 41 6 354 

125-198 ug/m3 3 9 9 43 121 285 112 216 102 66 53 92 101 23 1 1,236 

79-124 ug/m3 7 10 29 123 318 879 348 738 415 225 237 178 112 19 - 3,638 

50-78 ug/m3 18 20 48 230 535 1,704 793 1,558 941 512 522 301 76 13 - 7,271 

31-49 ug/m3 10 15 47 247 581 2,063 1,051 2,402 1,497 894 718 310 61 7 - 9,903 

19-30 ug/m3 13 7 45 145 369 1,449 866 2,186 1,512 908 689 227 39 5 - 8,460 

12-18 ug/m3 - 1 10 54 183 700 423 1,124 933 586 457 110 13 2 2 4,598 

7-11 ug/m3 3 1 5 19 87 245 162 469 375 239 192 52 9 1 - 1,859 

5-6 ug/m3 - 1 1 3 16 41 28 91 91 55 49 22 1 1 - 400 

3-4 ug/m3 - - 1 3 10 28 19 54 48 32 24 11 2 1 - 233 

1-2 ug/m3 - - - - 3 10 6 36 30 14 24 5 1 - - 129 

<1 ug/m3 - - - - 1 6 7 23 21 14 15 10 3 2 - 102 

TOTAL 55 66 199 889 2,260 7,509 3,851 8,958 5,985 3,562 3,000 1,376 538 163 27 38,438 

 PM10 Category < 1 1 mph 2 mph 3 mph 4 mph 5-6 7 mph 8-10 11-13 14-16 17-21 22-27 28-34 35-43 44+ TOTAL

1995-3161 ug/m3 - - - - - - - - - - - - - - - - 

1258-1994 ug/m3 - - - - - - - - - - 2 - - 7 - 9 

794-1257 ug/m3 - - 1 - - 3 1 - 1 - 3 1 12 14 5 41 

501-793 ug/m3 - 1 2 5 8 5 1 - 1 2 6 6 33 27 6 103 

316-500 ug/m3 - 1 1 25 74 94 11 19 4 2 5 25 59 25 6 351 

199-315 ug/m3 - 4 9 93 252 468 89 91 14 9 25 82 86 16 1 1,239 

125-198 ug/m3 1 4 31 214 562 1,054 242 358 90 42 70 181 103 13 1 2,966 

79-124 ug/m3 - 15 60 399 912 1,738 528 758 320 159 275 263 60 8 - 5,495 

50-78 ug/m3 - 29 63 393 951 1,897 721 1,399 732 470 654 275 47 3 1 7,635 

31-49 ug/m3 - 21 68 277 656 1,640 715 1,912 1,414 992 976 235 17 5 2 8,930 

19-30 ug/m3 - 20 27 138 287 780 461 1,511 1,443 1,006 714 117 13 3 1 6,521 

12-18 ug/m3 - 10 16 59 127 340 185 679 647 450 286 57 12 - - 2,868 

7-11 ug/m3 - 8 5 15 34 129 97 276 228 169 117 34 3 1 - 1,116 

5-6 ug/m3 - 1 1 7 10 24 19 57 50 28 21 9 - - - 227 

3-4 ug/m3 - - 1 1 7 14 9 35 35 25 23 7 - - - 157 

1-2 ug/m3 - 1 - - 3 12 2 10 22 10 11 2 - - - 73 

<1 ug/m3 - - - - 1 7 3 9 7 7 10 2 - - - 46 

TOTAL 1 115 285 1,626 3,884 8,205 3,084 7,114 5,008 3,371 3,198 1,296 445 122 23 37,777 

 PM10 Category < 1 1 mph 2 mph 3 mph 4 mph 5-6 7 mph 8-10 11-13 14-16 17-21 22-27 28-34 35-43 44+ TOTAL

1995-3161 ug/m3 - - - - 1 - - - - - - - - - 1 2 

1258-1994 ug/m3 - - 1 - - - - - - - - - - - - 1 

794-1257 ug/m3 - - - - - 3 2 - 1 - 2 - 2 5 3 18 

501-793 ug/m3 - - 1 2 5 4 - - - 4 7 4 10 16 4 57 

316-500 ug/m3 - - 7 39 57 34 13 9 4 1 7 18 29 22 3 243 

199-315 ug/m3 1 4 35 109 175 204 43 44 13 6 18 47 67 20 2 788 

125-198 ug/m3 2 4 90 355 490 608 164 186 64 29 61 94 84 15 2 2,248 

79-124 ug/m3 - 1 169 659 809 1,210 394 657 306 111 164 166 79 11 2 4,738 

50-78 ug/m3 - 3 127 634 909 1,670 660 1,514 838 474 501 315 60 5 1 7,711 

31-49 ug/m3 - 3 91 429 732 1,575 721 2,199 1,700 1,090 945 333 31 5 1 9,855 

19-30 ug/m3 - 2 31 159 313 809 498 1,685 1,631 1,207 946 209 18 3 - 7,511 

12-18 ug/m3 - - 11 47 109 275 197 740 779 571 457 75 9 3 - 3,273 

7-11 ug/m3 - - 5 16 36 110 92 325 282 217 150 25 2 2 - 1,262 

5-6 ug/m3 - - - 1 5 17 17 71 69 48 35 10 2 - - 275 

3-4 ug/m3 - - - 2 2 15 10 36 51 24 31 10 1 - - 182 

1-2 ug/m3 - - - 1 2 4 3 26 20 13 20 5 - - - 94 

<1 ug/m3 - - - - - 11 1 20 15 13 11 2 - - - 73 

TOTAL 3 17 568 2,453 3,645 6,549 2,815 7,512 5,773 3,808 3,355 1,313 394 107 19 38,331 

 PM10 Category < 1 1 mph 2 mph 3 mph 4 mph 5-6 7 mph 8-10 11-13 14-16 17-21 22-27 28-34 35-43 44+ TOTAL

3162+ ug/m3 - - - - - - - - - - - - - - 1 1 

1995-3161 ug/m3 - - - - - - - - - - - - - 2 2 4 

1258-1994 ug/m3 - - - - - - - - - - - - 1 - - 1 

794-1257 ug/m3 - - - - - - - 1 - - 1 - 4 5 3 14 

501-793 ug/m3 - - - 1 1 4 - 6 1 2 4 6 20 16 4 65 

316-500 ug/m3 - - 2 12 14 29 5 19 4 4 8 21 42 24 5 189 

199-315 ug/m3 - 1 4 26 50 113 50 99 40 12 17 51 52 16 2 533 

125-198 ug/m3 4 - 17 78 129 356 193 382 111 41 52 96 69 15 1 1,544 

79-124 ug/m3 5 2 30 127 255 862 423 898 289 115 156 162 76 14 1 3,415 

50-78 ug/m3 - 1 25 204 444 1,603 884 1,933 617 297 353 234 54 8 - 6,657 

31-49 ug/m3 3 - 30 187 510 1,755 1,145 2,760 1,354 815 647 297 40 4 - 9,547 

19-30 ug/m3 1 - 11 113 327 1,233 789 2,374 1,702 1,058 814 205 22 3 - 8,652 

12-18 ug/m3 2 - 3 54 153 622 422 1,210 940 664 426 96 8 2 1 4,603 

7-11 ug/m3 - - 4 24 68 290 187 562 391 270 149 42 6 2 - 1,995 

5-6 ug/m3 - - - 4 17 51 40 107 101 63 36 16 6 - - 441 

3-4 ug/m3 - - - 1 6 24 21 63 52 47 29 8 2 - - 253 

1-2 ug/m3 - - - 1 1 15 9 23 26 21 16 2 - - - 114 

<1 ug/m3 - - - - - 5 2 16 13 9 16 - - - - 61 

TOTAL 15 4 126 832 1,975 6,962 4,170 10453 5,641 3,418 2,724 1,236 402 111 20 38,089 

WIND GUST CATEGORY (MAXIMUM WIND GUST DURING HOUR - miles per hour)

BUCKEYE WEST 43RD AVE

Table 7. Crosstabulation of Hourly Maximum Wind Gust and PM Concentrations

DURANGO COMPLEX HIGLEY
15

9.0 Unusual Winds

For exceptional events purposes, “unusual winds” can be defined as any wind that has the ability to create 

wind blown dust. Literature and data from monitors indicate that the phenomenon of blowing dust can 

occur over a broad range, but generally is associated with hourly averaged wind speeds that are above 10 

miles per hour, which are commonly associated with wind gusts above 20 miles per hour. 

The National Weather Service (NWS) only includes “wind gust” information in the routine observations 

when the observed wind gust exceeds 15 miles per hour. As such, there is no bright-line indicator as to 

what constitutes unusual winds from the stand-point of a specific wind velocity, but the NWS generally 

uses 15 miles per hour as a significance point for wind gusts. 

The data analyzed in this study suggest that hourly averaged wind speeds greater than 10 miles per hour 

(which are commonly associated with wind gusts over 20 miles per hour) occur less than 5% of the time, 

and are thus, not common, and can be considered ‘unusual’ for exceptional events purposes. 

ADEQ’s analysis of tabular wind summaries, which use highlighting criteria of 15 and 25 miles per hour 

for the maximum winds during the hour, are consistent with the literature reviewed, the data analyzed, 

and the NWS criteria for significance of winds 
16

REFERENCES

Bach, A. J., Brazel, A. J., and Lancaster, N. 1996. Temporal and Spatial Aspects of Dust in the 

Mojave and Colorado Deserts of Southern California, 1973-1994. Physical Geography

17: 329-353. 

Brazel, A. J. and Nickling, W. G. 1986. The Relationship of Weather Types to Dust Storm 

Generation in Arizona (1965-1980). Journal of Climatology. 6: 255 -275. 

Gillette, D.A., Adams, J., Endo, E. and Smith, D., 1980. Threshold Velocities for Input of Soil 

Particles into the Air by Desert Soils, Journal of Geophysical Research 85(C10): 5621- 

5630. 

Gillette, D.A., Adams, J., Muhs, D. and Kihl, R., 1982. Threshold Friction Velocities and Rupture 

 Moduli for Crusted Desert Soils for the Input of Soil Particles into the Air.

Journal of Geophysical Research 87(C10): 9003-9015

Gillette, D.A., 1988. Threshold Friction Velocities for Dust Production for Agricultural Soils. 

Journal of Geophysical Research 93(D10): 12645-12662. 

Holcombe, T. L. Ley, T. and Gillette, D. A. 1996. Effects of Prior Precipitation and Source Area 

Characteristics on Threshold Wind Velocities for Blowing Dust Episodes, Sonoran 

Desert 1948-1978. Journal of Applied Meteorology. 36: 1160-1175. 

Lyles, L. 1988. Basic Wind Erosion Processes. Agriculture, Ecosystems, and Environment 

22/23: 91-101. 

MacKinnon D. J., Elder, D. F., Helm, P. J., Tuesink, M. F., and Nist, C. A. 1990. A Method of 

 Evaluating Effects of Antecedeant Precipitation on Duststorms and its Application to 

 Yuma, Arizona, 1981-1988. Climatic Change. 17: 331-360. 

McTainish G. H. and Pitblado, J. R. 1987. Dust Storms and Related Phenomena Measured from 

 Meteorological records in Australia. Earth Surface Processes and Landforms 12: 415- 

 424. 

Nickling, W. G. & Brazel, A.J. 1984. Temporal and Spatial Characteristics of Arizona Dust Storms 

(1965-1980). Journal of Climatology. 4: 645-660. 

Nickling, W. G. 1988. The Initiation of Particle Movement by Wind. Sedimentology 35: 499- 

 511. 

Nickling, W.G. and Gillies, J.A., 1989. Emission of Fine Grained Particulate from Desert Soils, 

in: Paleoclimatology and Paleometeorology: Modern and Past Patterns of Global 

Atmospheric Transport. M. Leinen and M. Sarnthein (Editors), Kluwer Academic 

Publishers, pp. 133-165.

Orgill, M. M. and Sehmel, G. A. 1976. Frequency and Diurnal Variation of Duststorms in the 

 Continental U.S.A. Atmospheric Environment 10: 310 

Wilkerson, W. D., 1991. Dust and sand forecasting in Iraq and adjoining countries, 

AWS/TN—91/001, Air Weather Service, Scott AFB, IL, 63 pp. 
17

WRAP, 2006. Fugitive Dust Handbook. http://www.wrapair.org/forums/dejf/fdh/. 

University Corporation for Atmospheric Research / COMET Program. 2003. Forecasting Dust 

Storms. http://www.meted.ucar.edu/mesoprim/dust/print.htm. 
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